In Type II diabetes, there is a progressive decline in insulin secretory function in beta cells in the face of ongoing insulin resistance. Currently available therapies are unable to prevent this decline (Diabetes 44:1249–1258, 1995; DeFronzo, Diabetes 37:667–687, 1988). Insulin resistance alone is not sufficient to cause Type II diabetes, and in fact, many individuals maintain insulin resistance for extended periods without becoming diabetic due to effective compensation by increased insulin secretion (Polonsky, Int J Obes Relat Metab Disord 24 Suppl 2:S29–31, 200). Insulin-resistant rats and mice display a compensatory increase in beta cell mass (Hribal, et al., Am J Physiol Endocrinol Metab 282:E977–981, 2002); the same phenomenon appears to occur in insulin resistant, but non-diabetic, (usually obese) humans (Kloppel, et al., Surv Synth Pathol Res 4:110–125, 1985; Butler, et al., Diabetes 52:102–110, 2003). In rodents, beta cell mass appears to be regulated by a changing balance between the positive effects of beta cell replication and neogenesis and the negative effects of beta cell apoptosis (Bonner-Weir, J Mol Endocrinol 24:297–302, 2000; Bonner-Weir, Trends Endocrinol Metab 11:375–378, 2000; Pick, et al., Diabetes 47:358–364, 1998; Finegood, et al., Diabetes 50:1021–1029, 2001). In humans, the onset of Type II diabetes due to insufficient increases (or actual declines) in beta cell mass is apparently due to increased beta cell apoptosis relative to non-diabetic insulin resistant individuals (Butler, et al., Diabetes 52:102–110, 2003). Agents which could specifically prevent this increase in beta cell apoptosis may therefore prevent insulin resistant individuals from developing Type II diabetes.
Beta cell death and apoptosis are also central to the onset of Type I diabetes, although the mechanisms that lead to loss of beta cell mass are primarily T-cell mediated in Type I and this is not the case in the majority of Type II cases (Mathis, et al., Nature 414:792–798, 2001). In Type I diabetes, recruitment and activation of T-cells and macrophages leads to an intra-islet environment rich in cytokines (interleukin (IL) 1-β interferon (IFN)-γ and tumor necrosis factor (TNF)-α), reactive oxygen species and nitric oxide (NO), each of which can promote beta cell apoptosis in vitro (Eizirik and Darville, Diabetes 50 Suppl 1:S64–69, 2001). Physiological beta cell apoptosis may actually trigger the immune response that results in wholesale islet destruction (Mathis, et al., Nature 414:792–798, 2001).
The mechanisms that lead to increased beta cell apoptosis are multiple and interlacing and they are as yet incompletely understood. Tumor necrosis factor (TNF)-α, which interacts with receptors TNF-RI and TNF-RII in both its membrane bound and soluble forms, can contribute to beta cell death in vitro (Kaneto, et al., Diabetes 44:733–738, 1995; Mandrup-Poulsen, et al., J Immunol 139:4077–4082, 1987). In the NOD mouse model of Type I diabetes, TNF-RI deficiency can prevent the onset of diabetes, presumably through reduction in beta cell death or apoptosis (Kagi, et al., J Immunol 162:4598–4605, 1999). Various modes of stress can also contribute to beta cell apoptosis (Zhou, et al., J Clin Invest 101:1623–1632, 1998).
Although there are likely to be apoptotic modalities that are relatively unique to the beta cell, there are some general mechanisms of programmed cell death that occur in many cell and tissues that form fundamental pathways for cytotoxic responses to UV irradiation, X-rays, thermal and osmotic shock, endoplasmic reticulum (ER) stress as well as the response to proinflammatory cytokines such as IL-1 beta and TNF-alpha. Some of these pathways are composed of cascades of mitogen-activated protein kinases (MAP kinases) (Kyriakis and Avruch, Physiol Rev 81:807–869, 2001). Cytotoxic stresses activate MAP kinase kinase kinases (MAPKKKs), which phosphorylate and activate MAP kinase kinases (MAPKKs), which in turn phoshorylate and activate MAP kinases such as ERK, JNK1-3 and p38 (Johnson and Lapadat, Science 298:1911–1912, 2002; Tibbles and Woodgett, Cell Mol Life Sci 55:1230–1254, 1999). JNKs, which phosphorylate and activate the transcription factor c-Jun among other substrates, are critical mediators of apoptosis (Tournier, et al., Science 288:870–874, 2000).
Apoptosis signaling kinase (ASK)-1/MAPKKK5 is a ubiquitously expressed component of the kinase cascade that activates JNK and p38 (Takeda, et al., Cell Struct Funct 28:23–29, 2003). ASK1 directly phosphorylates MKK4(SEK1)/MKK7 and MKK3/MKK6, which in turn phosphorylate the JNKs and p38 (Ichijo, et al., Science 275:90–94, 1997). A constitutively active form of ASK1 is obtained by truncating an N-terminal regulatory domain; expression of this active kinase leads to apoptosis via mitochondria-dependent caspase activation (Hatai, et al., J Biol Chem 275:26576–26581, 2000). Cells from mice that lack ASK1 are resistant to the apoptotic effects of oxidative stress and TNF-α (Tobiume, et al., EMBO Rep 2:222–228, 2001).
The role of ASK1 in oxidative stress-initiated apoptosis may be mediated in part by a direct physical interaction with the redox-regulatory protein thioredoxin (TRX) (Saitoh, et al., Embo J 17:2596–2606, 1998). Trx inhibits ASK1 kinase activity upon binding to the N-terminal domain that is lacking in the constitutively active form of ASK1. The interaction between ASK1 and Trx is dependent on Trx being in the reduced form; this provides a mechanism by which the redox state of the cell can regulate ASK1 kinase activity (Liu and Min, Circ Res 90:1259–1266, 2002). Accordingly, reactive oxygen species such as H2O2 cause dissociation of Trx-ASK1 complexes and lead to ASK1 activation (Gotoh and Cooper, J Biol Chem 273:17477–17482, 1998; Tobiume, et al., J Cell Physiol 191:95–104, 2002).
There is also evidence that ASK1 promotes apoptosis in cells undergoing endoplasmic reticulum (ER) stress. The ER protein IRE1 forms a complex with ASK1 and a TNF receptor interacting protein TRAF2 in cells undergoing ER stress, and this leads to activation of the ASK1-JNK pathway. The apoptosis initiated by this pathway is reduced in cells that lack ASK1 (Nishitoh, et al., Genes Dev 16:1345–1355, 2002).